Mattress innerspring units are generally formed of a plurality of spring coils arranged in side-by-side relation in parallel rows, with parallel columns also thereby being formed orthogonal to the rows. Border wires typically encircle both the upper and lower perimeters of the innerspring unit formed by the most outboard spring coils and are connected to terminal convolutions formed on the ends of the spring coils.
It is a common practice to form the terminal convolution with an enlarged diameter with respect to that of the spirals which are axially inward from the coil ends. This facilitates interengagement of the springs and makes the spring coil more stable in compression. Terminal convolutions of adjacent spring coils in a row are overlapped, and helical spring coils, referred to as cross-helicals, are then wound along columns to encircle the overlapped convolution portions. These crosshelicals ordinarily have an internal diameter which is slightly larger than the combined diameters of the overlapped terminal convolution portions. Larger diameter helical springs are also often used to attach a border wire to the terminal convolutions.
There are some general considerations of manufacture and comfort which underlie the design of any mattress innerspring. For example, considerable effort has been devoted in the industry to the development of terminal convolutions which facilitate the interengagement of the spring coils as well as their connection to the border wire. For example, terminal convolutions have been developed having offset portions formed thereon which include a straight part. This enables the spring ends to be secured along a substantial length of the straight part which will "catch" more helical spirals, and thereby provide more stability for the individual coils. Improved stability is always being sought, however.
These efforts have also been directed to find ways to prevent the spring coils from rotating relative to each other, and perhaps out of the upper or lower plane of the unit. A related problem in this area is that of hinging, where the overlapped portions of coils slip over one another and make noise. Hinging can further occur between coils overlapped with the border wire. Such hinging is obviously desired to be kept at a minimum in a mattress.
Another consideration in mattress design and manufacture is the ability to make innerspring units which have different firmness characteristics suited to an individual's personal preference. This may simply amount to providing several mattress lines having differing firmness, or, in more sophisticated mattresses, providing areas of different firmness in a particular mattress innerspring.
As may be readily recognized, producing mattresses with different firmness characteristics may be accomplished through the use of springs of differing compression for each mattress firmness, ordinarily achieved by making the various springs out of different wire stock or in different configurations. The overall layout or construction of the innerspring unit may also be changed from one mattress firmness to another, such as by changing the coil count and coil arrangement. Use of heavier wire stock, more springs, different springs or a different layout obviously adds expense to mattress production in terms of parts as well as labor. A primary consideration in making mattresses with different degrees of firmness is therefore to do so in the most efficient and economical manner while still achieving the desired results.